The present invention relates to techniques for the modification of sub-pixels.
The most commonly used method for displaying images on a color mosaic display is to pre-filter and re-sample the pixels of the image to the display. In the process, the R, G, B values of selected color pixels are mapped to the separate R, G, B elements of each display pixel. These R, G, B elements of a display pixel are sometimes also referred to as sub-pixels. Because the display device does not typically allow overlapping color elements, the sub-pixels can only take on one of the three R, G, or B colors. The color's amplitude, however, can be varied throughout the entire grey scale range (e.g., 0-255). Accordingly, a rendering that maps image pixels to display sub-pixels is performed.
Referring to FIG. 1A, there exists a number of variety of different sub-pixel configurations. In general, the sub-pixel combinations can be grouped as RGB striped, RGBW striped, multi-primary, or repeating two-dimensional patterns. For each sub-pixel configuration the associated “display” is shown as a 4×4 array of sub-pixels immediately below in FIG. 1B.
Active matrix liquid crystal display panels achieve their images, in part, because of the individual transistor and capacitor placed at each sub-pixel. The transistor and capacitor latch the data to the pixel electrode that controls the amount of backlight that passes through a given sub-pixel. Occasionally, one or more transistors will malfunction, resulting in one or more defective sub-pixels. There are at least two ways a transistor can fail. One failure mode, a permanently open circuited transistor, results in an always-off or always-on sub-pixel. Another mode of failure, a permanently short circuited transistor, results in a sub-pixel whose brightness value varies over time but in a way not directly tied to the image data to which it should be associated. Also, the sub-pixels may be stuck at an intermediate constant value or may vary in some manner based upon the state of the display, such as the data currently in the frame buffer.
Always-on sub-pixels appear as randomly placed red, blue, and/or green elements on an all-black background. Always-off sub-pixels appear as black or colored dots on all-white or colored backgrounds. The probability of always-on and always-off sub-pixel defects depends on the LCD process. In the most general case, a defective sub-pixel is a sub-pixel whose output light value can not be controlled.
By way of example, the data in the frame buffer may vary the pixel value when the row driver connection to the defective sub-pixel is damaged such that the sub-pixel is always “enabled.” In this case, as the scan lines are written to the column drivers, the signal to the faulty sub-pixels will fluctuate according to the instantaneous values in the column buffer for that column. The slow temporal response will tend to make the output of the defective sub-pixel a constant for the duration of (at least) a frame period. That constant is approximately given by
  f  ⁡      (                  ∑                  i          =          0                          N          -          1                    ⁢                        p          i                /        N              )  where pi is the signal input to the ith sub-pixel in the column containing the defective sub-pixel, N is the number of display lines, and f accounts for the temporal response of the sub-pixel and the transfer function between signal and light output. This value will generally be different from the desired output were the sub-pixel operating properly.
Referring to FIG. 2, an example of five 4×4 displays is illustrated with the always-off defective sub-pixels. It is shown that the defective sub-pixels are illustrated as black regions. In some cases, such as the right-hand configuration shown in FIG. 2, a white sub-pixel may be used to enhance the luminance of the display, without altering the color gamut. Referring to FIGS. 3A and 3B, the white sub-pixel may be used to correct for defects. As illustrated, the de-saturated portion of the pixel in FIG. 3A is transferred to the white sub-pixel in FIG. 3B. However, the same white sub-pixel, along with the additional “headroom” in the primary color pixels can be used to hide some defects in a given RGBW quadruple.
The foregoing and other objectives, features, and advantages of the invention will be more readily understood upon consideration of the following detailed description of the invention, taken in conjunction with the accompanying drawings.